1. Field of the Invention
The invention relates to a unit for controlling a plurality of hydraulic actuators, through which the actuators are supplied by a single flow-rate generator, each being connected to it through a proportional directional valve.
2. Description of the Prior Art
It is known that directional valves are appliances which are disposed between a flow-rate generator and an actuator in order to control the functioning of the actuator by adapting the way in which it is connected to the flow-rate generator.
Directional valves of the proportional type have not only a controlled spool, the position of which determines the cross section of a throttle, but also an automatic compensating spool for keeping constant the pressure difference between the upstream and downstream of this throttle, so that a given flow-rate of fluid corresponds to a given position of the controlled spool. Consequently, when an actuator is controlled with a proportional directional valve, its speed of operation is determined by the position of the controlled spool, independently of the load which the actuator bears.
When the flow-rate generator is used for supplying a plurality of actuators, with a proportional directional valve corresponding to each of them, it may happen that the total flow-rate demanded by the actuators exceeds the maximum flow-rate which the flow-rate generator is able to supply. The respective compensating spools are then no longer able to maintain, in each of the directional valves, the pressure difference between the upstream and downstream of the throttle at the predetermined constant, so that the most heavily loaded actuators slow down or stop while the less heavily loaded ones are able to continue to operate.
In order to avoid such malfunctioning, it has already been proposed to use the regulation of the flow-rate generator according to the power demanded, which at the present time is provided for on the majority of hydraulic circuits where the flow-rate generator supplies a plurality of actuators. This regulation is achieved by providing load-detection means known as "load sensing", which refer back to the flow-rate generator the pressure of the most heavily loaded actuator, to which pressure the flow-rate generator responds by producing a service pressure equal to the load-sensing pressure increased by a constant. In reality, this constant is added for as long as the total flow-rate demand is less than that capable of being supplied but, in the event of excessive demand, the value added to the load-sensing pressure is smaller than the constant, and the higher the excessive flow-rate demand, the smaller the value. It is this reduction in the value added which is used in order to avoid the aforesaid malfunctioning.
French Pat. No. 2,339,757 proposes to act on the actuating pressure of the controlled spool in each of the proportional directional valves, by arranging for the actuating valves to be fed not directly from a pilot pump but rather with the interposing, between the pilot pump and the actuating valves, of an operating margin valve which varies the supply pressure for the control valves in the same way as the difference between the pressure of the flow-rate generator and the load-sensing pressure. As long as the flow-rate generator is operating normally, the supply pressure for the control valves remains constant, just as if these valves are supplied directly by the pilot pump. In the event of an excessive flow-rate demand, the supply pressure for the control valves will decrease as a function of the magnitude of the excessive demand, the actuating pressure of all the controlled spools will decrease in the same way, and consequently all the throttlings produced by the controlled spools will increase in the same way, with the result that each directional valve will have applied to it the same level of flow-rate reduction so that all the actuators will slow down, with preservation of their speed ratio.
French Pat. No. 2,548,290 proposes to arrive at the same result where the proportional directional valves have a compensating spool which is located upstream of the controlled spool, by acting on the means of actuating the compensating spool: it continues to be forced in the direction of closing by the pressure upstream of the controlled spool and in the direction of opening by the pressure downstream of the controlled spool, but a double pressure force is substituted for the conventional spring, respectively in the direction of closing by the load-sensing pressure and in the direction of opening by the pressure of the flow-rate generator. The difference in pressure between upstream and downstream of the throttle of the controlled spool thus depends on the difference between the pressure of the flow-rate generator and the load-sensing pressure, which leads to the aforesaid result.
The invention aims to obtain this same result, but with improved performance.